10 - avoiding deadlock.rb

O Reilly Ruby Cookbook source code

require 'thread'
$chopstick1 = Mutex.new
$chopstick2 = Mutex.new

class Philosopher < Thread
  def initialize(name)
    super do
      loop do
        $chopstick1.synchronize do 
          puts "#{name} has picked up one chopstick."
          $chopstick2.synchronize do
            puts "#{name} has picked up two chopsticks and eaten a " +
                 "bite of tasty rice."
     end
        end
      end
    end
  end
end

Philosopher.new('Moore')
Philosopher.new('Anscombe')
# Moore has picked up one chopstick.
# Moore has picked up two chopsticks and eaten a bite of tasty rice.
# Anscombe has picked up one chopstick.
# Anscombe has picked up two chopsticks and eaten a bite of tasty rice.
# Moore has picked up one chopstick.
# Moore has picked up two chopsticks and eaten a bite of tasty rice.
# ...
#---
require 'thread'
pool_lock, lion_lock, penguin_lock, cabbage_lock = (1..4).collect { Mutex.new }
locks = [pool_lock, lion_lock, penguin_lock, cabbage_lock]
$lock_order = {}
locks.each_with_index { |lock, i| $lock_order[lock] = i }

def lock_all(*locks)
  ordered_locks = locks.sort_by { |x| $lock_order[x] }
  ordered_locks.each do |lock|
    puts "Locking #{$lock_order[lock]}." if $DEBUG
    lock.lock          
  end
  begin
    yield
  ensure 
    ordered_locks.reverse_each do |lock|
      puts "Unlocking #{$lock_order[lock]}." if $DEBUG
      lock.unlock
    end    
  end
end
#---
$DEBUG = true
lock_all(penguin_lock, pool_lock) do 
  puts "I'm putting the penguin in the pool."
end
# Locking 0.
# Locking 2.
# I'm putting the penguin in the pool.
# Unlocking 2.
# Unlocking 0.
#---
require 'thread'
$lock = Mutex.new
Thread.new do 
  $lock.synchronize { $lock.synchronize { puts 'I synchronized twice!' } }
end
#---